The present invention relates to an ink composition useful for a thermal ink jet printer or a thermal ink permeation printer, particularly to such that generates gas by relatively small amount of heat, and that causes no clog of nozzles of a thermal printer even after a long-term storage.
Thermal ink jet printers and thermal ink permeation printers have been well known as thermal printers that eject ink to a recording medium by heating. In the thermal printers, the ink is heated by heating elements to generate gas, thereby increasing an inner pressure of an ink fountain. According to the thermal ink jet printers, the ink is jetted by the increased inner pressure through nozzles onto a recording sheet, to provide an image on the sheet. According to the thermal ink permeation printers, the ink permeates through a porous film disposed between a thermal head and a recording sheet by the increased inner pressure, to provide an image.
Thus, the thermal printers eject the ink to a recording medium by gas-development from the ink, whereby the ink is required to generate gas by small amount of heat. Conventionally, aqueous inks having a low boiling point obtained by dissolving a water-soluble dye in water or in a mixed solvent of water and a hydrophilic solvent have been mostly used as the ink for the thermal printers.
The aqueous inks have advantage of generating gas by relatively small amount of heat. However, in the thermal printer using the aqueous inks, water composing the inks is often vaporized to provide a dried dye. The dried dye causes clog of nozzles of the printer, whereby the printer must be periodically cleaned. In such a case, because it is difficult to arrange a plurality of nozzles in one direction to form a line head, the thermal printer generally exhibits a low printing speed. Further, a printed matter obtained by such aqueous inks is poor in water resistance, so that deterioration such as stain of a printed image is caused by water.
Accordingly, an object of the present invention is to provide such an ink composition for a thermal ink jet printer or a thermal ink permeation printer that generates gas by relatively small amount of heat, that causes no clog of nozzles of a thermal printer even after a long-term storage, and that provide a printed matter excellent in water resistance.
As a result of intense research in view of the above object, the inventors have found that an ink composition as mentioned above is obtained by adding a heat decomposition gas-developing agent (blowing agent) to an oil-based ink. The present invention has been accomplished by this finding.
Thus, an ink composition of the present invention comprises: an oil-based ink composed of an organic solvent, and an oil-soluble dye and/or a pigment dissolved or dispersed in the organic solvent; and a heat decomposition gas-developing agent that is decomposed by heating to a decomposition temperature thereof to generate gas. Although conventional ink compositions using a nonvolatile solvent generally have a high boiling point, the ink composition of the present invention exhibits an apparent boiling point reduced by the heat decomposition gas developing agent. The heat decomposition gas-developing agent is decomposed to generate N2 gas, etc. by heating, whereby the ink composition of the present invention is ejected by a relatively small amount of heat, to be useful for the thermal ink jet printer or the thermal ink permeation printer. Further, when a preferred nonvolatile solvent is used as the organic solvent composing the oil-based ink, the nozzles of the thermal printer are not clogged with the ink composition even after a long-term storage, whereby above-mentioned periodical cleaning is simplified or omitted.